The present invention relates to cellular and other wireless communication systems, and more particularly, to methods and apparatus for determining the data rate in communication systems utilizing code division multiple access (CDMA) techniques.
Code division multiple access (CDMA) techniques have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another. Current CDMA networks were designed to carry only voice traffic and exhibit limited data rate variability. CDMA networks, however, must evolve to encompass a variety of multimedia applications, each having potentially different data rates. Thus, CDMA networks will be required to carry information associated with the various multimedia applications at various data rates, corresponding to the requirements of diverse wireless services demanded by customers.
Many communication systems utilizing code division multiple access (CDMA) comply with the IS-95 standard, adopted by the Telecommunication Industry Association (TIA). Under the IS-95 standard, a communication system substantially eliminates co-channel interference and improves the bit energy-to-noise density ratio, Eb/No, on the forward link from a base station or cell site to a mobile receiver unit by modulating the information signals with Walsh orthogonal function sequences. To produce corresponding orthogonal information signals, these CDMA systems require that the forward link information signals be transmitted in a synchronized manner. A more detailed discussion of the IS-95 standard is provided in xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d Telecommunication Industry Association Doc. No. TIA/EIA/IS-95 (1993), incorporated by reference herein.
Current implementations of the IS-95 standard allow a limited number of data rates. The Telecommunication Industry Association has recently adopted a new standard, IS-95B, for increasing the data rates that may be achieved with CDMA networks. The IS-95B standard permits data rates that are integer or power-of-two multiples of the basic CDMA rate. Thus, in one implementation, data rates (before error control coding) of 1,200 (1R), 2,400 (2R), 4,800 (4R) or 9,600 (8R) bits-per-second are available. In a second implementation, data rates (before error control coding) of 1,800 (1R), 3,600 (2R), 7,200 (4R) or 14,000 (8R) bits-per-second are available. All rates in a particular coding scheme come from only one rate set.
Typically, the higher data rates are utilized when there is intense voice activity. Thus, the highest data rates are utilized only when necessary, and the average data rates are reduced. The data rate changes frame by frame, with each speech frame being 20 milliseconds long. A mobile receiver, however, has no knowledge of what the data rate is for a given received frame. The IS-95 standard does not indicate the data rate of the transmitted signal. Thus, a mobile receiver has to detect the data rate before decoding the data, since decoding depends on the data rate. Accordingly, a need exists for a method and apparatus for determining the data rate of a received signal in a communication receiver utilizing code division multiple access (CDMA) techniques.
Generally, a method and apparatus are disclosed for determining the data rate of a received signal in a communication system utilizing code division multiple access (CDMA) techniques. The present invention combines the well-known Viterbi decoding-based rate detection approach with the conventional repetition pattern-based rate detection approach. The hybrid approach of the present invention possesses the advantages of both prior approaches, without their disadvantages.
The present invention recognizes that the computationally efficient repetition pattern-based data rate detection approach, while not as reliable as the Viterbi decoder-based data rate detection approach, provides reliable data rate detection most of the time. Thus, the present invention integrates the two prior art techniques, and utilizes the repetition-pattern data rate detection approach as long as a predefined reliability metric is satisfied, and only uses the more computationally intensive Viterbi decoder-based data rate detection approach when detection reliability may be compromised.
Under the hybrid rate detection scheme, a decision variable based on repetition patterns is initially formed and a decision is made on the data rate if the decision variable is in a reliable region. Otherwise, the Viterbi decoding-based rate detection scheme is employed. By properly establishing reliability thresholds, TH and TL, for each possible data rate, the hybrid scheme has the same or better performance as a Viterbi decoding-based rate detection scheme, while also exhibiting the superior computational efficiency of a repetition pattern-based rate detection scheme (since the Viterbi rate detection is only performed when required).
If a decision variable, D, based on repetition patterns is less than a first reliability threshold, TL, the data rate can reliably be said to be higher than the data rate associated with the decision variable. Thus, the hybrid scheme proceeds to evaluate the decision variable associated with the next highest data rate. Likewise, if the decision variable, D, is greater than a second reliability threshold, TH, the data rate can reliably be said to be the data rate associated with the decision variable. Finally, if the decision variable, D, is not less than a first reliability threshold, TL, and the decision variable, D, also is not greater than a second reliability threshold, TH, then the Viterbi decoding-based rate detection scheme is employed to select a data rate from among the current data rate and all higher data rates.